So basically 16 people were able to pick up the behest and the last member was JP so I could do nothing to explain the problem to him (I was leader). We killed all the mobs in the area and there were apparently 2 crabs left but they did not spawn. We ran all over the place looking for them before giving up and just disbanding. Luckily another JP member explained to the guy that got left out so that's all good but I'm still wondering what the heck was going on. I've been doing nothing but behest for the last 7 or so levels and have never experienced this. Has anyone else had this happen?

Exactly. The max is 15. We had all 15 in the party and one other outside the party who I could not invite. Trust me, they weren't in the cave. The 16th member picking it up somehow screwed the entire behest up.